Operators of existing wells often wish to track the evolution of the reservoir and/or to identify any previously missed opportunities for increased hydrocarbon production. Among the variety of logging tools available to operators for use in existing wells is the pulsed-neutron logging tool. This tool can provide measurements of formation porosity even from inside a cased well. It operates by generating pulses of neutron flux that radiate from the tool into the surrounding environment including the borehole and the formation. The neutrons entering the formation interact with atomic nuclei, scattering from them until they dissipate enough energy to be captured. The scattering rate (and hence the rate of energy loss) is generally dominated by the concentration of hydrogen atoms in the formation. As the presence of hydrogen is primarily attributable to the presence of water or hydrocarbon fluids, the rate of energy loss is indicative of the fluid-filled porosity of the formation.
Some tool designs derive the porosity measurement by counting the number of neutrons scattered back to the tool. Other tool designs attempt to derive a more refined measurement of the scattering and capture rates through the use of gamma ray detectors that count gamma rays emitted by the atomic nuclei after scattering and capture events. As the gamma ray energy varies based on the type of event and type of atom, the gamma ray detectors are usually configured to measure the energy associated with each counted gamma ray. Because an unexpected concentration of other elements (e.g., chlorine) can skew the measurements, the gamma ray detector measurements may be windowed to include only gamma rays having energies indicative of interactions with hydrogen nuclei and/or to exclude gamma rays having energies indicative of interactions with other elements. Still other tool designs employ a hybrid approach, using both neutron and gamma ray detectors to improve measurement accuracy.
Operators of existing wells further wish to track the condition of their wells. The well casing and any other tubular strings in the well are subject to deterioration of various kinds including corrosion. Since the well casing is not readily accessible for manual inspection, it is typically necessary to employ a casing inspection tool to determine the well's condition. Periodic maintenance inspections enable early detection of such determination and the implementation of corrective action to prevent unexpected shutdowns and improve the general overall efficiency of the operation.
Various methods are known for determining the extent of casing corrosion and otherwise monitoring the condition of the casing. As casing is generally formed of a ferromagnetic iron alloy, it can be probed with magnetic fields, eddy currents, visual inspections, and ultrasonic measurements, which can various provide measurements of resistivity, thickness, irregularities, and other indicators of defects. Generally such measurements require a logging run with a dedicated, single-purpose tool, creating an added and possibly unnecessary cost for the operator.
It should be understood, however, that the specific embodiments given in drawings and detailed description thereof do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and other modifications that are encompassed in the scope of the appended claims.